In diagnostic imaging modalities, such as X-ray, ultrasound and magnetic resonance (MR) imaging for example, the use of contrast agents to enhance contrast between different tissues or organs or between healthy and damaged tissue is a well established technique. For the different imaging modalities, contrast enhancement by the contrast agent is achieved in different ways. Thus in proton MR imaging for example contrast agents generally achieve their contrast enhancing effect by modifying the characteristic relaxation times (T.sub.1 and T.sub.2) of the imaging nuclei (generally water protons) from which the MR signal which is used to generate the image derives.
When injected into a living being, materials with magnetic properties such as paramagnetism, superparamagnetism, ferromagnetism and ferrimagnetism can cause a reduction in the T.sub.1 and T.sub.2 (or T.sub.2 *) values of tissue water protons. Although a reduction in T.sub.1 cannot occur without a reduction in T.sub.2 (or T.sub.2 *), the fractional decrease in T.sub.1 can be different from that in T.sub.2 (or T.sub.2 *). If the fractional decrease in T.sub.1 is greater than that in T.sub.2 (or T.sub.2 *) then the intensity of the MR image increases, and the material is referred to as a T.sub.1, or positive, contrast agent. If the fractional decrease in T.sub.1 is less than that in T.sub.2 (or T.sub.2 *) then the intensity of the MR image decreases, and the material is referred to as a T.sub.2 (or T.sub.2 *), or negative, contrast agent.
Particles with the magnetic properties of superparamagnetism, ferrimagnetism and ferromagnetism are referred to herein as magnetic particles.
The first suggestion in the literature for the use of magnetic materials as MR contrast agents was the proposal in 1978 by Lauterbur that manganese salts might be used in this regard. The first proposal in the patent literature was the suggestion by Schering in EP-A-71564 (and its equivalent U.S. Pat. No. 4,647,447) that chelate complexes of paramagnetic metal ions, such as the lanthanide ion Gd (III), might be used.
The early commercial contrast agents for MR imaging, such as GdDTPA, GdDTPA-BMA and GdHP-D03A available from Schering, Nycomed and Bracco under the trade marks MAGNEVIST, OMNISCAN and PROHANCE, are all soluble chelate complexes of paramagnetic lanthanide ions and in use are positive contrast agents which enhance image intensity from the regions to which they distribute.
Subsequently, particulate ferromagnetic, ferrimagnetic and superparamagnetic agents were proposed for use as negative MR contrast agents. Oral formulations of such particulate agents, generally referred to herein as magnetic particles, have become available commercially for imaging of the gastrointestinal tract, e.g. the product ABDOSCAN available from Nycomed Imaging. However parenteral administration of such particulate agents has also been widely proposed for imaging of the liver and spleen as these organs act to remove foreign particulate matter from the blood relatively rapidly. Thus, by way of example, lever and spleen imaging using such agents is proposed by Widder in U.S. Pat. No. 4,859,210.
More recently it has been proposed, for example by Pilgrimm in U.S. Pat. No. 5,160,725 and WO-94/21240, that the uptake of parenterally administered magnetic particles from the blood by the reticuloendothelial system may be hindered, and thus blood residence time prolonged, by chemically binding a stabilizer substance to the magnetic particle surface.
Examples of materials which may be used in this way as stabilizers include carbohydrates such as oligo- and polysaccharides, as well as polyamino acids, oligo- and polynucleotides and polyalkylene oxides (including poloxamers and poloxamines) and other materials proposed by Pilgrimm in U.S. Pat. No. 5,160,725 and WO-94/21240, by Nycomed in PCT/GB94/02097, by Bracco in U.S. Pat. No. 5,464,696 and by Illum in U.S. Pat. No. 4,904,479.
Magnetic particles coated in this fashion can then be used as blood pool agents (i.e. for imaging the vasculature) or for lymph node imaging, or alternatively they may be conjugated to biotargeting agents and used for imaging the targeted tissues or organs.
When administered as blood pool agents, it has been found with magnetic particles that the fractional reduction in T.sub.1 of the blood protons can be greater than the fractional decrease in T.sub.2 (or T.sub.2 *) and thus such agents can be used as positive MR agents for the vasculature.
For parenteral use, the size and size distribution of the composite particles and the chemical nature of the surface of the overall particle are of great importance in determining the contrast generation efficacy, the blood half-life, and the biodistribution and biodegradation of the contrast agent. Ideally the magnetic particle size (i.e. the crystal size of the magnetic material) is within the single domain size range (such that the particles are superparamagnetic and thus have no hysteresis and a reduced tendency to aggregate) and the overall particle size distribution is narrow so that the particles have uniform biodistribution, bioelimination and contrast effects. Preferably, the magnetic particles should be provided with a surface coating of a material which modifies particle biodistribution, e.g. by prolonging blood half-life, or by increasing stability, or which acts as a targeting vector causing preferential distribution to a target site, such as a tumour site.
Mean crystal sizes, i.e. of the magnetic core material, should generally be in the range 1 to 50 nm, preferably 1 to 20 nm and especially preferably 2 to 15 nm and, for use as blood pool agents, the mean overall particle size including any coating material should preferably be below 30 nm. Producing superparamagnetic crystals or composite particles having such sizes is not in itself particularly problematical. However producing particles with the desired size, acceptable size distribution and without undue crystal aggregation does represent a problem and it is to the solution of this problem that one aspect of the present invention is directed.
Typically, the magnetic crystals are produced by liquid phase precipitation, generally in a solution of a polymeric coating agent (e.g. using a co-precipitation technique such as that described by Molday in U.S. Pat. No. 4,452,773). This technique results in the generation of relatively polydisperse particles which require a subsequent size fractionation step, e.g. by centrifugation or chromatography. By way of example it is by such a technique that the product AMI 227 of Advanced Magnetics is produced.
We have now found that magnetic particles with particularly advantageous properties can be produced by precipitation in a branched polymer containing aqueous medium and subsequently cleaving the polymer to release composite particles comprising magnetic particles and a cleaved polymer coating.